1. Field of the Invention
The present invention relates to a mobile communication system such as a cellular phone, a base station control device and a radio base station forming the system, and a communication timing control method in the system.
2. Description of the Related Art
Mobile communication system whose representative is a cellular phone or a PHS is an information communication means indispensable in the information age of today. The mobile communication is realized by transmission and reception of radio signals by a mobile terminal to and from a radio base station and transmission and reception of wire signals by the radio base station to and from a network connecting to a communication target. Since one radio base station has its communicable area limited, a plurality of radio base stations form an area (cell) covering communication to compensate for each other, thereby enabling mobile communication everywhere. In a case where a mobile terminal conducts communication, it uses a radio base station responsible for communication in a cell to which it belongs. When the mobile terminal comes out of the area of the cell to which it originally belongs to move to other cell during communication, there arises the need of communication using a radio base station existing in the cell to which it has moved. At this time, if the terminal is allowed to use only one radio base station, communication is instantaneously cut off at the instant of switching to a radio base station to be used. For avoiding such problem, mobile communication systems of today employ a method of preventing communication from cutting off even when a mobile terminal moves among cells by enabling the mobile terminal to simultaneously communicate with a plurality of radio base stations when the mobile terminal is located near the boundaries of a plurality of cells.
For enabling this method, a plurality of radio base stations existing near a terminal should receive user data and timing should be controlled such that data from each radio base station to the terminal arrives at the terminal at the same time. A base station control device in general takes charge of this control and upon receiving user data to the terminal, the base station control device copies the data and controls timing of transmission to each radio base station such that the user data arrives at the plurality of radio base stations at the same time.
FIG. 13 is a conceptual diagram showing a relationship among a base station control device, a radio base station and a terminal in a common mobile communication system.
In FIG. 13, in the mobile communication system, user data C from a core network 1 is received by a base station control device 2, the received user data C is transmitted from the base station control device 2 to a plurality of radio base stations 3 (3-1, 3-2, 3—3) and user data B in question is sent from the plurality of radio base stations 3-1, 3-2 and 3—3 as user data A by radio, whereby the user data is transmitted from the relevant radio base station 3 to a mobile terminal 4.
In the above-described mobile communication system, for conducting communication timing control, the base station control device 2 in advance defines, as a fixed value, a time from the reception of the user data C by the base station control device 2 until the transmission of the user data as the user data A by the radio base station 3 by radio (the time period will be hereinafter referred to as a total delay time in RAN). In addition, at the start of communication, calculate the amount of transmission delay corresponding to a time period of data transmission between the base station control device 2 and the respective radio base stations 3-1, 3-2 and 3—3. Upon receiving the user data C from the core network 1, the base station control device 2 determines timing of transmitting user data to the respective radio base stations 3-1, 3-2 and 3—3 according to the following expression:(transmission time)=(reception time)+(total delay time in RAN)−(the amount of transmission delay between the base station control device and the radio base station).
With this timing control method, when the amount of transmission delay is small, the base station control device 2 needs to buffer user data for a time period approximate to a total delay time in RAN, which might invite an increase in the total amount of delay of the entire system. In addition, when the amount of transmission delay between the base station control device 2 and the radio base station 3 changes due to handover or the like, the radio base station 3 senses deviation of user data reception timing to instruct the base station control device 2 to modify transmission timing, thereby realizing timing modification. For realizing such control, however, timing should be synchronized between the radio base station 3 and the base station control device 2 and the radio base station 3 needs to monitor normality of timing of data reception from the base station control device 2 all the time.
Conventional timing control method will be more detailed with reference to the drawings. First, at the time of communication start, timing is adjusted such that user data simultaneously arrives at all the radio base stations 3 (3-1, 3-2, 3—3) taking the amount of transmission delay between the base station control device 2 and the respective radio base stations 3-1, 3-2 and 3—3 into consideration.
FIG. 14 is a timing chart showing a conventional timing control method at the time of communication start.
Timing is adjusted such that at the respective radio base stations 3-1, 3-2 and 3—3 connected with the base station control device 2 through a transmission path (cable), a relationship between a sequence number applied to a frame (hereinafter referred to as CFN) and radio transmission timing of frame is the same.
To the respective radio base stations 3-1, 3-2 and (3—3), the base station control device 2 transmits a synchronous frame with a CFN appropriately applied. Each of the radio base stations 3-1, 3-2 and (3—3) having received the synchronous frames transmitted from the base station control device 2 responds to the base station control device 2 with a time difference between the time when it received the synchronous frame and timing when a frame having the CFN applied to the synchronous frame is transmitted.
Based on the difference received from each of the radio base stations 3-1, 3-2 and 3—3, the base station control device 2 adjusts a relationship between the CFN to be applied to the frame and the frame transmission timing and adjusts user data sending timing such that the frame arrives meeting the CFN transmission timing of each of the radio base stations 3-1, 3-2 and 3—3.
FIG. 15 is a timing chart showing timing of transmission of user data to each of the radio base stations 3-1, 3-2 and 3—3 at the time of down frame reception in the conventional method.
As a fixed value, determine in advance the above-described total delay time in RAM from when the base station control device 2 receives user data from the core network 1 until when the radio base stations 3-1, 3-2 and 3—3 transmit the user data by radio.
Upon receiving the user data from the core network 1, the base station control device 2 adjusts timing of transmission of user data to each of the radio base stations 3-1, 3-2 and 3—3, that is, each buffering time to be applied, such that the user data arrives at the radio base stations 3-1, 3-2 and 3—3 after a lapse of the total delay time in RAM after the reception of the user data.
When the amount of transmission delay between the base station control device 2 and the radio base stations 3-1, 3-2 and 3—3 changes due to hard handover or the like, timing is modified.
FIGS. 16 and 17 are timing charts showing the conventional timing control method at the time of modifying the timing. FIG. 16 shows a state before the timing is modified, while FIG. 17 shows a state after the timing is modified.
The radio base stations 3-1, 3-2 and 3—3 have a window having a fixed time span for each CFN transmission timing and when receiving a frame which can not be accommodated in the window, report deviation from the window to the base station control device 2.
Upon receiving the report, the base station control device 2 adjusts timing of user data transmission to the radio base station 3 by as much as the deviation from the window.
As described in the foregoing, since in such a conventional timing control method as described above, time is defined in advance from when the base station control device 2 receives data until when the radio base station 3 transmits the data and based on the time, a buffering time of the base station control device 2 is determined, data should be buffered for a long period of time at the base station control device.
In addition, since timing control between the radio base station and the base station control device is complicated to make it difficult to reduce a delay time of user data transmission in the system as a whole.
Under these circumstances, proposed for a mobile communication system in which a signal processing device is provided between a base station control device and a core network are the technique (Japanese Patent Laying-Open (Kokai) No. 2001-333446) of preventing deterioration of communication quality by reducing time of delay in transmission from the base station control device to the signal processing device, thereby reducing the number of user frames aborted due to delayed arrival and the techniques (Japanese Patent Laying-Open (Kokai) No. 2001-16159 and Japanese Patent Laying-Open (Kokai) No. 2001-358638) of maintaining communication quality by measuring a signal propagation time (transmission delay time) of a signal between a mobile station moving fast and a radio base station and conducting appropriate control according to the measurement result.
These techniques, however, fail to have an intention to suppress an increase in the scale of the base station control device, while reducing a user data transmission time in the system as a whole by reducing a user data (frame) buffering time in the base station control device.